This disclosure generally relates to non-destructive inspection (NDI) equipment and methods, and deals more particularly with methods and apparatus for inspecting a soft-tooled hollow structure, especially an elongated hollow structure made of composite material.
A variety of elongated composite structures may have relatively confined internal cavities that require inspection in order to assure that the structure meets production and/or performance specifications. One known elongated composite structure with tapering internal cavities is an integrally stiffened wing box for an airplane. One example application is in the horizontal stabilizer of an aircraft. A horizontal stabilizer structural box may be fabricated as a large co-cured structure that requires the use of soft internal tools to facilitate tool removal after the cure. If a given co-cured composite structure is considered primary structure, it would therefore need to be inspected to ensure structural integrity. Inspecting large soft-tooled composite structures presents four distinct yet interrelated challenges.
(1) Access: The interior of the part is often inaccessible to conventional ultrasonic scanning systems. Equipment and techniques are needed to transport the ultrasonic probes through the interior of the structure.
(2) Coverage: The entire interior surface of the structure needs to be inspected. While equipment and techniques can be used to transport ultrasonic probes to all parts of the structure in a global sense, there is a further need for specialized positioning hardware and techniques to ensure that locally, complete coverage is provided.
(3) Rate: The production manufacturing of structure for an active airplane program needs to be done at a rate that meets schedule commitments. Inspection of primary structure is a necessary part of the manufacturing process and must be done at a rate capable of keeping up with the published schedule.
(4) Conformity: Soft-tooled composites as opposed to hard-tooled composites have surfaces, particularly filleted join regions (referred to herein as “radii”), that are not precisely defined. That is, a designed nominal radius of 0.5 inch will, after fabrication, end up with a “radius” that is not truly circular, being approximated by a spline curve, but possibly with a radius that varies from its nominal value. This is an effect of the soft-tooled fabrication process. In addition, this manufacturing process does not turn out “radius” surfaces that are identical from radius to radius or from part to part. There is inherent variation in the manufacturing process that the ultrasonic inspection equipment and techniques need to take into account. This fabrication variation has special implications for ultrasonic inspection of composite structure because to get acceptable inspection of the interior of the composite part, the ultrasound beam should enter normal or near normal to the front surface of the part. This is because if the beam strikes the front surface at an angle, it will be refracted off normal and a return echo from any possible internal structure or anomaly will not occur. This normality requirement is complicated by the inherent part variations, which means that to ensure sound normality, NDI equipment and techniques should be designed to accomplish this.
Accordingly, there is a need for a system for inspecting the interior of a wing box and similar elongated hollow structures that has the foregoing capabilities.